Load sensing and power application circuit

ABSTRACT

A manually operable switch is operated to render conductive a first bidirectional current-conducting semiconductor device in a sensing circuit when a load is connected between output terminals to condition a power application circuit for supplying alternating current to the output terminals and to energize a relay and close contacts to render conductive a second bidirectional current conducting device and to complete the power application circuit to the load. Disconnection of the load from the terminals return the bidirectional current conducting semiconductor devices to a nonconductive state and disables the load sensing and power application circuit until a load is connected across the terminals and the switch is operated.

O United States Patent [111 3,566,198

[72] Inventor Kevin Barrie Delahunty 5 R f r Cited C m" UNITED STATESPATENTS [211 P ig 3,413,486 12/1968 Flieder 307/125 [221 FM 3,484,62312/1969 Cain 307/305 [45] Patented Feb. 23, 1971 [73] Assignee TeletypeCorporation Primary Examiner-James D. Trammwell Assistant Examiner-Harvey Fendelman Attorneys-J. L. Landis and R. P. Miller ABSTRACT: Amanually operable switch is operated to render conductive a firstbidirectional current-conducting semiconductor device in a sensingcircuit when a load is con- [54] LOAD SENSING AND POWER APPLICATIONnected between output terminals to condition a power appli- CIRCUIT In Fcation circuit for supplying alternating current to the output 16 cumsterminals and to energize a relay and close contacts to render [52] US.Cl 317/33, conductive a second bidirectional current conducting device307/305, 307/252 and to complete the power application circuit to theload. [51] Int. Cl "02h 7/00, Disconnection of the load from theterminals return the H03k 17/56 bidirectional current conductingsemiconductor devices to a [50] Field of Search 307/125, nonconductivestate and disables the load sensing and power 126, 130, 131,252, 202,305,305 (A); 317/33; application circuit until a load is connectedacross the ter- 340/256; 322/ l l minals and the switch is operated.

i3 3/ 37-2 /3 g 49 i LOAD SENSING AND POWER APPLICATION CIRCUITBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to load sensing and power application circuits and, moreparticularly, to a circuit which senses the presence of a load tocondition a power application circuit for subsequently supplyingalternating current power from a source to the load which is connectedacross output terminals.

2. Technical Consideration and the Prior Art The continuous growth ofthe employment of electrically operated equipment in thetelecommunication field has engendered problems in the supply of powerfor such equipment. Accordingly, considerable technical effort has beenexpended to develop circuitry to supply power for a variety of loads. inthe telecommunication art, it is extremely important to provide acertain amount of safety for personnel in the operation of equipmentwhich is made available in commercial offices. It is most important thata safety control circuit be interposed between a source of alternatingcurrent and the equipment or load so that when the load is disconnectedfrom a pair of terminals of the circuit, the source of current isdisconnected from the terminals.

It is therefore an object of this inventionto provide a loadsensingcircuit which senses-thepresence of a load to condition a powerapplication circuit for supplying alternating current power to the load.

One of the characteristics that must be considered in the design of anyparticular power circuit is the cost factor which may be further brokendown into the initial cost of construction, the operating cost, and themaintenance cost. Additional characteristics include space requirementsof the circuit, and I the reliability, sensitivity, and efficiency withwhich the circuit functions to accomplish the desired operation.

Since power control circuits may be used to control the duration ofpower application to a load, the response of the power control circuitto the connection of a load across output terminals of the circuit is ofextreme importance. Moreover, it is desirable to supply a maximum amountof power to the load with a minimum amount of dissipation in the controlcircuitry. Power dissipation may be attributed to the utilization ofcertain components and to the number of components in the controlcircuit.

It is therefore an object of this invention to provide a control circuitfor supplying power to a load wherein a minimum number of reliable,sensitive, and efficient components is utilized and wherein each of thecomponents is characterized by relatively low power consumption.

' SUMMARY or THE INVENTION With these and other objects in mind, thepresent invention contemplates a circuit which senses the presence of aload connected across output terminals and conditions a powerapplication circuit and operates-a switching device to complete thepower application circuit to supply power to the load, and which isrendered ineffective to apply power when the load is removed from theterminals.

More particularly, a sensing circuit for supplying alternating currentfrom a source to output terminals, includes a first bidirectionalcurrent conducting switching device which is rendered conductive when aload is connected across the terminals and a manually operated switch isdepressed. The presence of the load also controls the conditioning of apower application circuit. The switching of the first bidirectionalcurrenbconducting device to a conductive state energizes a relay toclose contacts to render conductive a second bidirectionalcurrent-conducting switching device and to bypass the switch which isnow released to complete the power application circuit. When the load isremoved from the terminals, the switching devices are renderedinoperative to deenergize the relay and open the power applicationcircuit and effectively disconnect the surce from the output terminals.

Other objects and advantages of the present invention will be apparentfrom the following detailed description when considered in conjunctionwith the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION Referring now tothe accompanying FIGURE, there is shown a system or circuit 10 forsensing the presence of a load 15 which may be connected across a pairof output terminals 11 and 12, and for then coupling the outputterminals through a three-prong plug 13 to a source 14 of alternatingcurrent. The center prong of the plug 13 has a conductor 16 connected toground 17.

A load-sensing circuit extends from the source 14 through the plug 13and a conductor 18 through a normally closed, manually operated switch19 and on through a normally open, manually operated switch 21 and thenalong the conductor 18 through a junction point 24, through a winding 26of a relay 27 and then on through a first bidirectional currentconducting semiconductor device, designated generally by the numerals28, through a conductor 29 to a junction point 31 and back along aconductor lead 32 through a circuit breaker 33 to the plug 13. -Atrigger or control circuit for the first bidirectional currentconducting semiconductor device 28 may be traced from junction point 24,through a load connected between the terminals. 12 and 11, over a lead36, through a junction point 37, over a lead 38, through a resistor 39to a control electrode or gating terminal 41 for the device 28.

The bidirectional current conducting semiconductor device 28 may be anyof several devices well known in the art, e.g., pairs of electronictubes or silicon-controlled rectifiers connected in parallel withopposing orientation. Control circuitry is used to selectively deliverindependent triggering pulses to the gating terminals in order toachieve a desired operation of the circuit.

Another type of electronic switching device that may be used is onedesignated in the trade by the name triac. Triac is a generic term thathas been used to identify a triode AC semiconductor device. Thesesemiconductor devices normally exhibit a high-impedance characteristicbetween two main current-carrying terminals. When a relatively lowpower-triggering impulse is imparted to a gating or third terminal, thedevice is switched to a second state wherein a low impedance isexhibited between the main current-carrying terminals.

Moreover, the triggering impulse which is imparted to the gatingterminal to switch the device from a high to a low impedance state maybe of either polarity. When one of the current-carrying terminals ispositive with respect to the other current-carrying terminal, the triacmay be switched to a lowimpedance current conductive state byapplication of a triggering current to the gating terminal. On the otherhand, when the reverse polarity is applied between the main terminals,that is when the other current-carrying terminal is negative withrespect to the one current-carrying terminal, the device may be switchedto a low impedance current-carrying state by extracting a triggeringcurrent from the gate terminal. ln other words, conduction of currentfrom the other main terminal to the one main terminal is initiatedby theapplication of a positive gating pulse to the gating terminal whereascurrent conduction from the one main terminal to the other main terminalis initiated by the application of a negative triggering pulse to thegating terminal.

Hence, the bilateral characteristic of the main current-conducting pathand the flexibility offered by the nonrestrictive form of triggeringimpulse render the bidirectional currentconducting semiconductor deviceswell suited for utilization in control circuits for supplyingalternating current power. A typical bidirectional current-carryingsemiconductor device,

as generally referred to in thejartas triac, is described in detail 1 inU.S. Pat. 3,275,909 issued to FQW. Gutzwiller, filed Dec. 19, 1963 andassigned to theGeneral Electric Company. A typical application of atriac to apower control circuit is described in U.S. Pat.3,346,744-issued to E. K. Howell, filed Feb. 7, 1964 and assigned to theGeneral Electric Company.

As shown, the triac 28 has a first 'main terminal 42 connected through atransconductive path to a second main terminal 43. The triac 28 isfurther provided with the gating electrode or terminal 41 which iseffective when suitably energized to switch the transconductive pathinto a conductive or low-impedance state. Once the conductive state isestablished, the gating terminal 41 has no further effect on the triac28 until the nonconductive state is reestablished by removing the biasfrom the transconductive path. When the main terminal 42 is positiverelative to the main terminal 43, the triac 28 is rendered operable byapplying apositive gating pulse to the gate terminal41. Whenthe reversepolarity is applied between the main terminals 42 and 43,the device isswitched to a low impedance state by applying a negative triggeringpulse to the control electrode 41. I

The system also includes a power application circuit which extends fromthe source 14 and plug 13 along the conductor lead 18, through thenormally closedswitch19, through a pair of normally open contacts 271associated with the relay 27, through the junction point 24 and to theterminal 12 on through the load 15 and terminal 11 back along theconductor lead 36 through the junction point 37 and a secondbidirectional current-conducting semiconductor device or triac,designated generally by the numerals 46. Finally, the power applicationcircuit extends from the second triac 46 through a pair of normally opencontacts 27-2, associated with the relay 27, through the junction point31 along the con-- ductor 32 through the circuit breaker 33 to the plug13 and source 14. i

The second bidirectional curre nt conducting semiconductor device ortriac 46, identical to the first current-conducting semiconductor device28, includesa main terminal 47 connected to the junction point 37,. asecond main terminal 48 connected to the relay contacts- 27-2 and gatingor third ter- 21, AC voltages are impressed'on the junction points 24and 31. Assume that the voltage impressed at junction point 24 andterminal 42 is positive going and that the voltage impressed at junctionpoint 31 and terminal 43 is negative going; then, the positive goingvoltage is also impressed through the load 15, over the leads 36 and 38through the resistor 39 to the gating terminal 41 of the triac 28.

The application of a positive going potential to the gating terminal 41of the triac 28, when the terminal 43 is negative with respect to theterminal 42 switches the triac to a conductive state to enable thetran'sconduction path between the main terminals 43 and 42. When thetriac 28 is switched to a low-impedance state between main terminals,the load-sensing circuit path from the plug 13 through the conductor 18and switches 19 and 21, then through the relay winding 26 and triac 28and conductor 29 to the junction point 31 and then back through theconductor 32 to the source 14 is completed to energize the relay 27.

It can be appreciated that when the alternating current source 14impresses a negative going potential over the conductor 18 through thewinding 26 toapply a negative going potential to the terminal 42 of thetriac 28, that a positive going pulse is applied over the conductors 32and 29 to apply a positive potential to the terminal 43. At the sametime, a negative going voltage is impressed through the junction point24 and load 15 through the junction point 37 and along the leads'36 and38 through the resistor 39 to apply a negative potential to the gatingterminal41. The application of the negative potential to the gatingterminal 41 when the terminal 43 is positive relative to the terminal'42 switches the triac 28 to conductive state.

in this way, the triac 28 is maintained in a conductive state eventhough the source 14 alternately supplies negative and positive goingpulses over the conductors. Although, the change in potential on theterminals 42 and 41 from positive I to negative and the changein-potential on the terminal 43 minal 49. A trigger or control circuitfor second triac extends from the gating terminal 49 through a resistor51 to the junction point 37. v v v OPERATION operator that the circuit10 is supplied with alternating current power.

The operator next depresses the switch 21 and AC voltage is impressed bythe source 14 along the conductor 18 through the normally closedmanually operated switch 19 and the now closed manual switch 21 throughthe junction point 24 and relay winding 26 of the relay 27 to make theterminal 42 of the triac 28 positive. At the same time, an AC voltage isimpressed along the conductors 32 and 29 and applies a negative goingpotential on the other main terminal 43 0f the triac 28.

in the event that no load is connected across the output terminals 11and 12, the triac 28 is not operated and power application circuit isnot rendered effective and when theoperator releases the switch 21, thecircuit remains inoperative in the original condition with the pilotlight 53 illuminated to indicate availability of -cycle alternatingcurrent power.

On the other hand, if the load 15 is. connected across the terminals 11and 12 when the operator depresses the switch from negative to positivetends to return the triac to a nonconductive state, the bidirectionalcharacteristic of the triac reestablishes the conductive pathinterconnected between the main terminals to maintain the device in aconductive state.

The applicationof AC voltage along the conductor 18 and through the load15 and junction point 37 also is effective to apply positive andnegative going potentials to both the main terminal 47 of the triac 46and through the resistor 51 to bias the gating terminal 49 whereupon the.power application circuit, serially connected through the mainterminals 48 and 47 of the triac 46, is conditioned for subsequentoperation.

The energization of the relay 27 closes normally open contacts 27-1 tolock in a circuit path through the conductor 18 to the load 15 after theoperator releases the manually operated switch 21. Also, the'energization of the relay 27 closes normally open contacts 27-2 toapply negative and positive going potentials to themainterminal 48 ofthe triac 46. Inasmuch as positive and negative going potentials areapplied to the other main terminal 47 and the gating terminal 49,

' the second triac 46 is rendered conductive to apply power throughthe-conditioned power ."application circuit from the source to the load15.

Since the triac' 28 is conductive only so long as the gating terminal 41is biased by positive and negative going voltages impressed through theload 15 connected across the terminals 11 and 12, the removal of thatload immediately returns the triac 28 to a nonconductive state whereuponthe relay 27 is deenergized. The deenergization of the relay 27 opensthe contacts 27-2 and interrupts the current through the powerapplication circuit. Alternatively, the normally closed, manuallyoperated switch 19 may be depressed to open the switch and interrupt thecircuit through the conductor 18 to the load 15 to deenergize the triac28.

in addition, the removal of the load 15 from the terminals 11 and 12removes the application of positive and negative going potentials to thegating terminal 49 of the second triac 46 to return the second triac toa nonconductive state. The

second triac 46 provides additional safety for the circuit in that whenthe load is removed from the terminals 11 and 12, the triac 46 isimmediately rendered nonconductive. In this way, the discontinuation ofthe application of power across the terminals 11 and 12 need not awaitthe relatively slow release of the relay 27 upon switching of the firsttriac 28 to a nonconductive state.

The interruption of the circuit to the load 15 which deenergizes thetriac 28 and deenergizes the relay 27 also opens the normally open, nowclosed contacts 27-1. The opening of the 10 contacts 27-1, interruptsthe circuit through the conductor 18 which may not be again completeduntil the switch 21, now open, is again depressed.

In this way, after the load 15 is removed from across the terminals 11and I2 and the power application circuit is rendered ineffective, anoperator who inadvertently grasps the terminals 11 and 12 is notinjured, since it is necessary to depress the switch 21 before the loadsensing and power application circuit may be recornpleted.

lclaim:

1. In a circuit for controlling the application of a source of AC powerto a load:

a pair of output terminals across which the load is connected; and

means responsive to the connection of the load across both of theterminals for applying AC power from the source to both of saidterminals, and responsive to removal of the load for interrupting theapplication of power to both of said output terminals.

2. In a circuit for controlling the application of a source of AC powerto a load as set forth in claim 1, wherein said responsive meansincludes:

a bidirectional AC switching devicehaving a pair of main terminals and agating terminal;

means for connecting the main terminals across the source of AC power;

means for applying the potential applied from the AC source through theload and gating terminal to operate said switching device;

a relay having energizing terminals and having contacts interconnectingthe source of AC power and both said output terminals; and

means for connecting the relay energizing terminals across the source ofAC power to energize the relay.

3. In a circuit interconnecting a source of AC power and a load:

a selectively operable means for applying power to said load when saidoperable means is in an operated state and for precluding theapplication of power to the load when said operable means is in anunoperated state;

a bidirectional electronic AC device having two main terminals and agating terminal for initiating conduction between said main terminals inboth directions;

means connecting said main terminals across the AC power source, one ofsaid terminals being connected between said selectively operable meansand the AC power source;

means connecting said gating terminal and one of said main terminalsacross said load for initiating conduction of said electronic AC device;and

means responsive to the conduction of said device for operating saidprecluding means to apply power from said source to said load.

4. In a circuit interconnecting a source of AC power and a load as setforth in claim 3:

said selectively operable means is a pair of nonnally open contacts; and

said means for operating said selectively operable means is a relay.

5. In a circuit interconnecting a source of AC power and a load as setforth in claim 3, wherein:

said selectively operable means includes:

normally disabled switching means; and a second bidirectional electronicA.C. device.

6. In a circuit interconnecting a source of AC load as set forth inclaim 5, wherein:

said bidirectional electronic device is a first triac; and

said second bidirectional electronic device is a second triac havingmain current-conducting terminals connected serially with said normallydisabled means and having a gating terminal connected to said initiatingmeans.

7. In a circuit interconnecting a source of AC load as set forth inclaim 6, wherein:

said normally disabled switching means are normally open contacts; and

said operating means is a relay which is energized by the conduction ofsaid first triac for closing said normally open contacts and for openingsaid contacts when said first triac is rendered nonconductive.

8. In a load sensing and power application circuit interconnectedbetween a source of AC power and an electrical load:

a bidirectional switching device having two main currentcarryingterminals interconnected by a normally disabled path through the load'which is enabled by the application of a triggering potential to athird control electrode;

means for connecting the main current carrying terminals across thesource of AC. power;

a bias circuit connected across said load for applying triggeringpotential to the control electrode to enable the path and initiateoperation of the device;

a relay having normally open contacts interposed between V the powersource and the load; and

means connecting said relay to said device for operating said relay toclose said contacts to apply power from the source to the load.

9. An electrical circuit for sensinga load connected to output terminalsand for supplying power from a source to said terminals which comprises:

thyratron-like switching means having a control electrode and normallyexhibiting a high impedance between two main current carrying terminalsand exhibiting low impedance between said terminals upon application ofa control signal to said control electrode;

means for connecting said two main terminals in a currentcarrying pathto said load and said source;

normally open means interposed between said source and said switchingmeans for interrupting said current-carrying path;

means including the load for applying operating potential to one of saidmain terminals and applying control signals to said control electrode;and

means responsive to the presence of the load for closing saidinterrupting means to apply operating potential to the other of saidmain terminals to complete said current path to said load.

10. In a circuit for supplying alternating current from a source to aload which is connected across a pair of spaced terminals:

a relay having normally open contacts interposed between the source andthe load;

a bidirectional current conducting semiconductor device normallyexhibiting a high impedance characteristic between two maincurrent-carrying terminals thereof and exhibiting a low-impedancecharacteristic in response to the application of a control signal to athird, gating terminal thereof and the application of opposite voltagesto the main terminals, one of said main terminals connected to saidnormally open contacts and the other of said main terminals connected tosaid load;-

means connected from the gating terminal through the load to the sourcefor applying a biasing potential to the gating terminal and an operatingpotential to said other of said terminals connected to said load; and

means for energizing said relay to close said contacts to applypotential to the one main terminal and operate the semiconductor deviceto supply power to said load.

11. In a circuit fonapplying AC power from a source to a load:

power and a power and a a first triac having a gating terminal and mainterminals interconnected between the AC power and the load;

a normally open contact interposed between the AC power source and oneof said main terminals;

means for applying conditioning potential through the load to the gatingterminal; 7

a second triac having a gating terminal and main terminals connectedacross the source with the gating terminal of the second triac connectedto receive operating potential through the load, to render said secondtriac conductive; and

a relay coil energized by said second triac for closing said opencontact to render said first triac conductive to apply said AC power tosaid load. g

12. In a circuit for applying a voltage from a source of alter natingcurrent power to a load: s

a bidirectional current conducting semiconductor device normallynonconductive between two main current-carrying terminals thereof andrendered conductive between the two main terminals in response to theapplication of a control signal to a gating terminal thereof;

means for connecting said two terminals in a first currentcarrying pathto said source;

a second current-conducting path for supplying power to the load;

normally unoperated switching means in said second current-conductingpath for conducting current from the source to the load;

a relay in said first current-carrying path and having normally opencontacts interposed in said second currentconducting path between thepower source and the load;

and

means for applying a control signal to the gating terminal to rendersaid first device conductiveto energize said relay and close saidcontacts to operate said switching means and complete said secondconducting path from the source to the load.

13. In an electrical circuit for sensing a load connected to outputterminals and for supplying power from a source to said terminals whensaid load is connected thereto:

circuit means for connecting said source to said load;

a first triac having two main terminals connected in parallel with saidlead and a third, gating terminal connected in series with said load;

a second triac in said circuit means and having two main terminalsconnected in series with said load and a gating terminal connected inseries with the gating terminal of the first triac; I

normally unoperated switching means in said circuit;

means applying a triggering currentto said gating terminals of saidfirst and second triacs for switching said first triac to a conductivestate and for conditioning said circuit means for conductance; and

means connected in series with said first triac and energized by saidfirst triac for operating said normally unoperated switching means tooperate said. second triac and to complete said circuit means to saidload.

14. in an electrical circuit for sensing a load connected to outputterminals and for supplying power to said terminals when said load isconnected thereto:

first thyratron-like switching means having a control electrode, and anormally disabled transconductive path which is enabled by energizingthe control electrode; means for connecting said source to said loadwhich includes: second thyratron-like switching means identical to saidfirst-switching means; and i j normally open means interposed betweensaid source and load; means connected through the load for energizingsaid control electrodes to enable the path in the second-switching meansand condition the connecting means for conductance; and Y meansresponsive to the enabling of the path in the firstswitching device forclosing said normally open means to complete said connecting means.

15. In a system for sensing a load connected to output terminals and forthen applying alternating current power from a source to the load:

normally unconditioned circuit means'for connecting said source to saidload, including:

a first AC switching device having a gating terminal and a normallydisabled transconductive path between two main terminals which isenabled for conductance by applying a triggering currentof eitherpolarity to the gating terminal and a potential across the mainterminals; and first normally open means interposed between said sourcev and one of said main terminals; second normally open means interposedbetween said-load and said source; and circuit means connected to saidsource for sensing a load connected to the output terminals and forcompleting said normally unconditioned circuit means, which includes: Ii a second AC switching devicehaving a gating terminal seriallyconnected with the gating terminal of the firstswitching means fornormally applying potential from the source across the two mainterminals thereof;

means energized by said second-switching device for closing saidnormally open means; and

means responsive to the connection of a load to the output terminals forapplying potential to said gating terminals to condition said normallyunconditioned circuit means andto operate said second-switching deviceto energize said closing means and close said normally open means tooperate said first-switching device and complete. said now conditionedcircuit means.

16. In an electrical circuit for sensing a load connected to outputterminals and for supplying power from an AC source to said terminals:

a first bidirectional current conducting semiconductor device normallyexhibiting a high impedance disabled path between two maincurrent-carrying terminals and a low-impedance characteristic to enablesaid disabled path in response to the application ofa control signal toa third gating terminal thereof, the minimum value of said signal beingthe same for both directions of current between said two terminals; r

means for connecting said two main terminals of said first semiconductordevice in a current-carrying path to said source; 7

a normally unconditioned circuit between said load and said source whichincludes:

.a second bidirectional current-conducting semiconductor deviceidentical to said first semiconductor device with the gating terminal ofsaid second device connected in series with the gating terminal of saidfirst device and the two main terminals of the second device connectedin said normally unconditioned circuit from the source to the load;

normally open means interposed between said source and said secondsemiconductor device;

means for applying a control signal to said gating terminals to enablesaid paths and condition said circuit between said source and said loadfor applyingpower to the load; and

means responsive to the enabling of said path through said firstsemiconductor device for closing said normally open means to completethe conditioned circuit to supply power to the load.

1. In a circuit for controlling the application of a source of AC powerto a load: a pair of output terminals across which the load isconnected; and means responsive to the connection of the load acrossboth of the terminals for applying AC power from the source to both ofsaid terminals, and responsive to removal of the load for interruptingthe application of power to both of said output terminals.
 2. In acircuit for controlling the application of a source of AC power to aload as set forth in claim 1, wherein said responsive means includes: abidirectional AC switching device having a pair of main terminals and agating terminal; means for connecting the main terminals across thesource of AC power; means for applying the potential applied from the ACsource through the load and gating terminal to operate said switchingdevice; a relay having energizing terminals and having contactsinterconnecting the source of AC power and both said output terminals;and means for connecting the relay energizing terminals across thesource of AC power to energize the relay.
 3. In a circuitinterconnecting a source of AC power and a load: a selectively operablemeans for applying power to said load when said operable means is in anoperated state and for precluding the application Of power to the loadwhen said operable means is in an unoperated state; a bidirectionalelectronic AC device having two main terminals and a gating terminal forinitiating conduction between said main terminals in both directions;means connecting said main terminals across the AC power source, one ofsaid terminals being connected between said selectively operable meansand the AC power source; means connecting said gating terminal and oneof said main terminals across said load for initiating conduction ofsaid electronic AC device; and means responsive to the conduction ofsaid device for operating said precluding means to apply power from saidsource to said load.
 4. In a circuit interconnecting a source of ACpower and a load as set forth in claim 3: said selectively operablemeans is a pair of normally open contacts; and said means for operatingsaid selectively operable means is a relay.
 5. In a circuitinterconnecting a source of AC power and a load as set forth in claim 3,wherein: said selectively operable means includes: normally disabledswitching means; and a second bidirectional electronic A.C. device. 6.In a circuit interconnecting a source of AC power and a load as setforth in claim 5, wherein: said bidirectional electronic device is afirst triac; and said second bidirectional electronic device is a secondtriac having main current-conducting terminals connected serially withsaid normally disabled means and having a gating terminal connected tosaid initiating means.
 7. In a circuit interconnecting a source of ACpower and a load as set forth in claim 6, wherein: said normallydisabled switching means are normally open contacts; and said operatingmeans is a relay which is energized by the conduction of said firsttriac for closing said normally open contacts and for opening saidcontacts when said first triac is rendered nonconductive.
 8. In a loadsensing and power application circuit interconnected between a source ofAC power and an electrical load: a bidirectional switching device havingtwo main current-carrying terminals interconnected by a normallydisabled path through the load which is enabled by the application of atriggering potential to a third control electrode; means for connectingthe main current carrying terminals across the source of A.C. power; abias circuit connected across said load for applying triggeringpotential to the control electrode to enable the path and initiateoperation of the device; a relay having normally open contactsinterposed between the power source and the load; and means connectingsaid relay to said device for operating said relay to close saidcontacts to apply power from the source to the load.
 9. An electricalcircuit for sensing a load connected to output terminals and forsupplying power from a source to said terminals which comprises:thyratron-like switching means having a control electrode and normallyexhibiting a high impedance between two main current carrying terminalsand exhibiting low impedance between said terminals upon application ofa control signal to said control electrode; means for connecting saidtwo main terminals in a current-carrying path to said load and saidsource; normally open means interposed between said source and saidswitching means for interrupting said current-carrying path; meansincluding the load for applying operating potential to one of said mainterminals and applying control signals to said control electrode; andmeans responsive to the presence of the load for closing saidinterrupting means to apply operating potential to the other of saidmain terminals to complete said current path to said load.
 10. In acircuit for supplying alternating current from a source to a load whichis connected across a pair of spaced terminals: a relay having normallyopen contacts interposed between the soUrce and the load; abidirectional current conducting semiconductor device normallyexhibiting a high impedance characteristic between two maincurrent-carrying terminals thereof and exhibiting a low-impedancecharacteristic in response to the application of a control signal to athird, gating terminal thereof and the application of opposite voltagesto the main terminals, one of said main terminals connected to saidnormally open contacts and the other of said main terminals connected tosaid load; means connected from the gating terminal through the load tothe source for applying a biasing potential to the gating terminal andan operating potential to said other of said terminals connected to saidload; and means for energizing said relay to close said contacts toapply potential to the one main terminal and operate the semiconductordevice to supply power to said load.
 11. In a circuit for applying ACpower from a source to a load: a first triac having a gating terminaland main terminals interconnected between the AC power and the load; anormally open contact interposed between the AC power source and one ofsaid main terminals; means for applying conditioning potential throughthe load to the gating terminal; a second triac having a gating terminaland main terminals connected across the source with the gating terminalof the second triac connected to receive operating potential through theload, to render said second triac conductive; and a relay coil energizedby said second triac for closing said open contact to render said firsttriac conductive to apply said AC power to said load.
 12. In a circuitfor applying a voltage from a source of alternating current power to aload: a bidirectional current conducting semiconductor device normallynonconductive between two main current-carrying terminals thereof andrendered conductive between the two main terminals in response to theapplication of a control signal to a gating terminal thereof; means forconnecting said two terminals in a first current-carrying path to saidsource; a second current-conducting path for supplying power to theload; normally unoperated switching means in said secondcurrent-conducting path for conducting current from the source to theload; a relay in said first current-carrying path and having normallyopen contacts interposed in said second current-conducting path betweenthe power source and the load; and means for applying a control signalto the gating terminal to render said first device conductive toenergize said relay and close said contacts to operate said switchingmeans and complete said second conducting path from the source to theload.
 13. In an electrical circuit for sensing a load connected tooutput terminals and for supplying power from a source to said terminalswhen said load is connected thereto: circuit means for connecting saidsource to said load; a first triac having two main terminals connectedin parallel with said load and a third, gating terminal connected inseries with said load; a second triac in said circuit means and havingtwo main terminals connected in series with said load and a gatingterminal connected in series with the gating terminal of the firsttriac; normally unoperated switching means in said circuit; meansapplying a triggering current to said gating terminals of said first andsecond triacs for switching said first triac to a conductive state andfor conditioning said circuit means for conductance; and means connectedin series with said first triac and energized by said first triac foroperating said normally unoperated switching means to operate saidsecond triac and to complete said circuit means to said load.
 14. In anelectrical circuit for sensing a load connected to output terminals andfor supplying power to said terminals when said load is connectedthereto: first thyratron-like switching means having a cOntrolelectrode, and a normally disabled transconductive path which is enabledby energizing the control electrode; means for connecting said source tosaid load which includes: second thyratron-like switching meansidentical to said first-switching means; and normally open meansinterposed between said source and load; means connected through theload for energizing said control electrodes to enable the path in thesecond-switching means and condition the connecting means forconductance; and means responsive to the enabling of the path in thefirst-switching device for closing said normally open means to completesaid connecting means.
 15. In a system for sensing a load connected tooutput terminals and for then applying alternating current power from asource to the load: normally unconditioned circuit means for connectingsaid source to said load, including: a first AC switching device havinga gating terminal and a normally disabled transconductive path betweentwo main terminals which is enabled for conductance by applying atriggering current of either polarity to the gating terminal and apotential across the main terminals; and first normally open meansinterposed between said source and one of said main terminals; secondnormally open means interposed between said load and said source; andcircuit means connected to said source for sensing a load connected tothe output terminals and for completing said normally unconditionedcircuit means, which includes: a second AC switching device having agating terminal serially connected with the gating terminal of thefirst-switching means for normally applying potential from the sourceacross the two main terminals thereof; means energized by saidsecond-switching device for closing said normally open means; and meansresponsive to the connection of a load to the output terminals forapplying potential to said gating terminals to condition said normallyunconditioned circuit means and to operate said second-switching deviceto energize said closing means and close said normally open means tooperate said first-switching device and complete said now conditionedcircuit means.
 16. In an electrical circuit for sensing a load connectedto output terminals and for supplying power from an AC source to saidterminals: a first bidirectional current conducting semiconductor devicenormally exhibiting a high impedance disabled path between two maincurrent-carrying terminals and a low-impedance characteristic to enablesaid disabled path in response to the application of a control signal toa third gating terminal thereof, the minimum value of said signal beingthe same for both directions of current between said two terminals;means for connecting said two main terminals of said first semiconductordevice in a current-carrying path to said source; a normallyunconditioned circuit between said load and said source which includes:a second bidirectional current-conducting semiconductor device identicalto said first semiconductor device with the gating terminal of saidsecond device connected in series with the gating terminal of said firstdevice and the two main terminals of the second device connected in saidnormally unconditioned circuit from the source to the load; normallyopen means interposed between said source and said second semiconductordevice; means for applying a control signal to said gating terminals toenable said paths and condition said circuit between said source andsaid load for applying power to the load; and means responsive to theenabling of said path through said first semiconductor device forclosing said normally open means to complete the conditioned circuit tosupply power to the load.